The present invention relates to self-powered radiation detectors in which no drive potential is needed to produce a radiation flux indicative signal. These detectors have two spaced, electrically insulated, electrodes of conductive materials which have differing response to neutron and/or gamma radiation flux levels, such as are encountered within nuclear reactors. The typical self-powered detector is a relatively small diameter tubular member with a central emitter electrode and a coaxial outer collector electrode spaced from the emitter by insulating means.
The emitter electrode material is typically a high neutron cross section material for a neutron detector, while the collector electrode material is a low neutron cross section material. An electrical charge difference is developed across these electrodes as the result of the differing neutron capture capability of the emitter and collector and consequent electron generation and migration across the insulating means. This electron charge flow is externally sensed as being indicative of a function of neutron flux. For a gamma flux detector, the same structure and general principles apply, but with the materials being selected for their differing gamma response. In almost all cases, the outer collector electrode is a hermetically sealed member of high temperature resistant, low neutron cross section, metal or alloy, such as the high nickel content steel, Inconel. Inconel is a trademarked material of the International Nickel Co.
The emitter electrode material is selected for its radiation interaction capability, and two of the most commonly used materials have been rhodium and cobalt. These materials do not have particularly advantageous mechanical ductility and are rather brittle. This greatly complicates the fabrication of reliable detectors.
The conventional fabrication technique for such self-powered detectors involves starting with a coaxial body of relatively large diameter and gradually reducing the dimensions of the detector and electrode thickness by repeated swaging. On each swaging step the detector assembly is passed through a smaller die set until the desired detector dimensions are reached. In this multiple swaging step fabrication process, the central emitter, if it is rhodium or cobalt, or other such less ductile material, often breaks apart into electrically isolated segments so as not to be usable as a detector.
In U.S. Pat. No. 3,940,627, a self-powered detector is described with a cylindrical emitter electrode of neutron responsive material, with a metal sheath or tube of non-neutron responsive material such as stainless steel about the emitter material. This metal sheath controls the gamma energy initiated delayed beta current component to the detector current for a more accurate neutron signal from the detector.
In copending application, Ser. No. 909,418, filed May 25, 1978, entitled "Compensated Self-Powered Neutron Detector" the self-powered detector includes a shield layer on the outside of a cylindrical emitter electrode, and a shield layer on the inside of a coaxial tubular collector electrode. A gamma flux responsive self-powered radiation detector with a tubular emitter electrode with insulating means with the tubular emitter is taught in copending application, Ser. No. 911,578, filed June 1, 1978, entitled "Gamma Flux Responsive Self-Powered Detector With A Tubular Emitter".